For too long, Google has denied reality, only allowing you to sign in to a single account at a time -- despite the fact that many people operate multiple Gmail accounts. You might have separate work and personal accounts, for instance, and checking both of them has traditionally been a tad annoying.
There are work-arounds to signing in and out over and over, of course. You can use Internet Explorer's New Session or InPrivate browsing to launch multiple Gmail accounts, or you could just open multiple browser windows. Now there's a better way.
Finally, Google has implemented multiple account support. To turn this on -- it's off by default -- click Settings and then click Accounts and Import. At the bottom of the page, click Google account settings. You'll see the Personal Settings page where you can switch Multiple Sign-in from Off to Edit.
You'll have to click a few more checkboxes -- Google treats multiple accounts like powering up a nuclear bomb, requiring multiple acknowledgments -- and then you're in business.